It is typical in a spinning machine to provide a drafting frame which can extend all along a side of that machine, over a large number of spinning stations, with pairs of rollers successively traversed by the roving from an inlet pair to an outlet pair, the rollers being driven at progressively higher peripheral speeds so that the roving is drawn to the desired fineness.
Such a drafting frame may comprise lower rollers which extend the full length of the drafting frame and cooperate with upper rollers which can be mounted on weighting or loading arms, the latter pressing the upper rollers against the lower rollers and clamping the roving between the upper and lower rollers of the respective pairs. The upper rollers can be individual to a particular spinning station and, if desired, the upper rollers can be twinned, with one upper roller of each twinned roller arrangement, being provided on each side of the arm. The continuous rollers, i.e. the lower rollers which extend the full length of the drafting frame, may be milled or otherwise modified in the regions juxtaposed with the upper rollers to increase their grip on the roving which is to be drafted.
The roving, after drafting, passes to a spinning unit which may be a ring-spinning station or a pot-spinning station or the like where a twist is imparted to that roving and the roving can be wound up in a yarn body, e.g. a bobbin in the case of a ring-spinning apparatus or a yarn cake in the case of a pot-spinning station.
It is known from German patent document DE 44 26 249 A1 to subject a roving at the outlet side of a drafting frame to the suction action of a suction roller provided as a lower roller beneath the stretching field plane of the drafting frame and thus engaging the roving from below. This suction roller or drum, over the top of which the roving passes, serves to compact the roving and to densify it. For this purpose a perforation trace is provided in the suction roller and in the region in which the roving passes over it so that fibers are drawn toward the trace.
It will be understood that the roving emerges from the drafting frame at a certain width and upon passing onto the suction roller is drawn inwardly toward the perforation trace and is thereby compacted. The compacted roving then passes from the suction roller to the twist-imparting unit. As noted, the latter can as a rule be a ring-spinning station in which a spindle cooperates with a ring surrounding the spindle and on which a traveler orbits, the yarn passing through the traveler and being wound on the spindle from the traveler. The twist-imparting unit and the unit collecting the yarn body can, however, also be a pot-spinning station.
The suction roller or drum has a larger diameter than the remaining drafting frame rollers since it is hollow and in its interior may receive a shield which is nonrotatable but which confines the suction to a limited portion of the periphery of the drum, namely, that portion over which the roving lies in contact with the drum.
For example, when the suction drum is provided as a bottom roller and the twist extends back to the nip between the suction drum and a counter roller, the contact of the twist with the arcuate drum surface can be excessive. The hairiness of the yarn which is ultimately produced, i.e. the number of fibers which jut out from the condensed roving and the twisted yarn, the number of yarn breaks which arise and the overall quality of the yarn are all adversely affected by the fact that the suction roller or drum is a large diameter body and particularly of a diameter greater than the diameter of the other lower rollers of the frame. It has also been found that problems arise in the delivery of the roving into the zone in which twist is imparted thereto. The problem appears to be due to the fact that yarns whose fibers have staple lengths like those of cotton, i.e. of about 25 mm to about 40 mm, require drafting rolls, at least at the output of the drafting frame with diameters of 25 to 35 mm and this could not be achieved in the earlier systems described with such suction rollers or drums as the last lower roller from which the roving passed before receiving the twist.